Copper indium gallium (di)selenide (CIGS) is a I-III-VI2 semiconductor material composed of copper, indium, gallium, and selenium. The material is a solid solution of copper indium selenide (often abbreviated “CIS”) and copper gallium selenide. It has a chemical formula of CuInxGa(1-x)Se2 where the value of x can vary from 1 (pure copper indium selenide) to 0 (pure copper gallium selenide). CIGS is a tetrahedrally bonded semiconductor, with the chalcopyrite crystal structure. Its bandgap varies continuously with x from about 1.0 eV (for copper indium selenide) to about 1.7 eV (for copper gallium selenide).
Optoelectronic devices, such as photovoltaic devices (e.g., solar cells), require an optical absorber that also provides sufficiently long minority carrier lifetimes to enable the collection of the minority carriers by the electrodes in the device's structure without excessive recombination. In all semiconductor materials minority carrier lifetimes are dependent on the defect structure of those materials. The control of defect structure is critical to the successful manufacture of photovoltaic devices.
Thin film optical absorbers are more economical than thick film absorbers or coatings because they require a smaller amount of the precursor materials than thick films or coatings. CIGS are well-suited to thin film solar cells since they can be deposited on flexible substrate materials, producing highly flexible, lightweight solar panels. Various techniques have been proposed in the art for forming CIGS absorption layers for photovoltaic devices such as solar cells. Currently, most CIGS deposition is done either using co-evaporation or using selenization of metal precursors, both of which have their disadvantages. For example, co-evaporation has proved difficult to effectively commercialize because it is difficult to uniformly evaporate Cu, In and Ga metal elements over a wide area. Moreover, the melting point of copper is 1084° C., which leads to high process costs and affects the substrate temperature. For this reason, sputtering techniques for selenization of metal precursor are more widely used for production. However, the reaction pathway cannot be controlled with these selenization techniques, resulting in copper selenide compounds forming after selenization and/or uneven gallium distribution, which decreases device performance. Further sulfurization is required to finish the surface passivation. Still further, this technique requires large amounts of toxic and costly H2Se gas or a large amount of Se.
An alternative technique using release liners is described in U.S. Pat. No. 6,559,372 to Stanbery.